A conventional LED module includes a substrate, a wiring pattern formed on the substrate, and an LED chip (light emitting element) mounted on the substrate. The wiring pattern includes a main surface electrode formed on a main surface of the substrate, and the main surface electrode has a die pad portion for supporting the LED chip. The LED module is formed by sequentially performing a die bonding process of mounting the LED chip on the die pad portion and a wire bonding process of connecting the LED chip and the wiring pattern with wires.
FIG. 19A is a view for explaining the die bonding process. The die bonding process is performed using an existing die bonder. Specifically, an adhesive paste (not shown) such as silver paste is applied on a die pad portion 911 of a wiring pattern 910 formed on a substrate 900. The adhesive paste may be also applied to the bottom of an LED chip 920. Then, an adsorption collet 930 is used to place the LED chip 920 on the die pad portion 911. Thereafter, the LED chip 920 is bonded to the die pad portion 911 by reflowing the adhesive paste or the like in a heating furnace.
FIG. 19B is a view for explaining the wire bonding process. The wire bonding process is performed using an existing wire bonder. Specifically, first, a tip portion of a wire 950 is extracted from a capillary 940 of a wire bonder and is melted. Then, the melted tip portion is pressed against an electrode pad on the upper surface of the LED chip 920 (first bonding step). Next, the capillary 940 is moved while pulling out the wire 950 from the capillary 940, and the wire 950 is firmly pressed against the upper surface of a wire bonding portion 912 of the wiring pattern 910 and cut by the capillary 940 (second bonding step). As a result, the electrode pad on the upper surface of the LED chip 920 and the wire bonding portion 912 are electrically connected by the wire 950. FIG. 19B shows a case where the first bonding is performed on the electrode pad on the upper surface of the LED chip 920 and then the second bonding is performed on the wire bonding portion 912. However, on the contrary, the first bonding may be performed on the wire bonding portion 912 and the second bonding may be then performed on the electrode pad on the upper surface of the LED chip 920. Typically, a pressing force is stronger in the second bonding than in the first bonding.
FIGS. 20A to 20C are views illustrating a problem caused when mounting the LED chip 920 on the substrate 900. In the die bonding process described above, the LED chip 920 is arranged at a preset position on the die pad portion 911, but misalignment may occur depending on the accuracy of the die bonder which is a die bonding device. Due to this misalignment, the LED chip 920 is sometimes placed on the outside of the die pad portion 911 (see FIGS. 20A and 20B). At this time, when the electrode exists above a portion protruding from the die pad portion 911, there is a possibility that the bonding of the LED chip 920 is peeled off or shifted due to a stress applied by wire bonding (indicated by a solid arrow in FIG. 20C). In particular, in the case of a two-wire type LED chip having two electrode pads on its upper surface, one of the electrode pads is often disposed at an edge in a plan view, which makes such a problem noticeable. In the conventional LED module, for the purpose of avoiding this problem, the die pad portion 911 is uniformly formed to be large in all directions from the outer periphery of the LED chip 920 in a plan view. Therefore, the size of the LED module in a plan view has to be increased.